1. Field of the Invention
The invention generally relates to a zoom lens system which has a zoom ratio 3.0, and in particular to an digital camera and the like.
2. The Related Art
The traditional zoom lens system chiefly has two lens groups which are negative lens group and positive lens group. The zoom lens system generally has a zoom ratio about 2. It is obvious that the zoom ratio is too small, even the structure of the zoom lens system is simple. Further, the exit pupil distance of the lens system is too short. Especially, the illumination in the corner of image will be insufficient when applied to a digital camera and the like.
A zoom lens system is disclosed in U.S. Pat. No. 6,304,389, the zoom ratio is about 2.5. When it is applied in digital camera, the zoom ratio is still not sufficient. Further, the varied focal length lens system is comprises of three lens groups. When the focal length is changed, the three lens groups are moved respectively along the optical axis. Therefore, it is difficult to control the movement of the lens groups. Moreover, there is no design for cutting off infrared rays in the zoom lens system, the quality of image will be decreased.
An object of the present invention is to provide a zoom lens system which not only extends the exit pupil distance and improves the illumination in the corner of image, but also reduces the interference of infrared rays, in order to enhance the quality of image, simplify the structure, reduce the difficulty of controlling, and decrease the demands of manufacturing technology.
To achieve the above objective, the present invention provides a zoom lens system. In order from the object side, the zoom lens system comprises a first lens group having negative refractive power, a second lens group having positive refractive power and a third lens group having positive refractive power. The first lens group composes a first lens unit having negative refractive power and a second lens unit having positive refractive power, wherein the first lens unit includes a negative lens and a negative plastics lens having at least one aspherical surface, and the second lens unit includes a positive lens. The second lens group composes an aperture element, a first lens unit having positive refractive power, a second lens unit having positive refractive power and a third lens unit having negative refractive power, wherein the first lens unit includes a positive lens, the second lens unit is constructed by a cemented lens consisting of a positive lens and a negative lens, and the third lens unit includes a negative lens having at least on aspherical surface. The third lens group includes a positive lens unit with a multi-layer coating in the surface facing to the object side in order to cut off infrared rays and decrease the influence of infrared rays on image. Conventional infra red cut off filter is an additional piece. By designing a zoom lens we should reserve space to put the infra red cut off filter. If we use a multi-layer coating, which was integrated into a lens unit, the length of camera lens will become shorter. Since the third lens group is fixed, the exit pupil distance is extended and the illumination in the corner of image is improved. Further, there is lens having aspherical surface in the first lens group and the second lens group, as a result, excellent correction is made for various aberrations especially spherical aberration, coma astigmatism. Herewith a good imaging quality is obtained.